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Higgs boson glimpsed at work for first time

By Lisa Grossman

HARD at work, Higgs? A rare kind of particle interaction at the Large Hadron Collider has given us our first glimpse of the Higgs boson doing what the Higgs boson does.

The Higgs was dreamed up to explain why some force-carrying particles such as the W and Z bosons have mass, while others such as the photon do not. For 50 years, it was the missing piece in the standard model of particle physics, which predicts how fundamental particles interact.

The ATLAS experiment at the LHC near Geneva, Switzerland, was one of the detectors that helped discover the Higgs in 2012. Now ATLAS physicists report seeing pairs of W bosons bumping into each other inside the detector (arxiv.org/abs/1405.6241).

This rare process can be used to test how the Higgs actually operates, and perhaps open paths to new physics, says Marc-André Pleier at Brookhaven National Laboratory in New York.

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Finding the effect at the LHC has been a challenge because W bosons scatter off each other incredibly rarely, even less often than a Higgs boson is produced. The collider smashes protons together at close to the speed of light. Every so often, one of those protons will emit a W boson. We see scattering only if both protons happen to emit a W boson at the same time, and if those W bosons happen to be aimed at each other.

But how is the Higgs involved? When theorists tried to calculate how often W bosons should interact with each other, the results were physically impossible – unless the particles toss a Higgs boson between them as they collide.

This makes W boson collisions one of the best places to look for physics beyond the standard model, which we suspect is incomplete because it does not take gravity into account and cannot explain mysteries such as dark matter and dark energy. If W bosons can exchange more than one Higgs during an interaction, for example, they should fly off each other, or scatter, more often than the standard model predicts.

“The rates of these scattering processes and the energies you see them at would be forced to change fairly dramatically,” says Matthew Herndon at the University of Wisconsin Madison, who works on another LHC experiment called CMS.

ATLAS has seen evidence for 34 of these events among billions of collisions, says Pleier, and for now, everything fits with standard model predictions. But seeing the effect at all is a milestone, and Herndon says the CMS experiment will also be releasing its version of these results soon.

ATLAS has seen evidence for 34 of these rare events among billions of collisions

“We’ve never looked in this corner of the standard model before,” says ATLAS team member Jake Searcy at the University of Michigan, Ann Arbor. “This is the start of something that’s going to be very interesting.”

This article appeared in print under the headline “First sighting of the Higgs at work”